Combine all ingredients for the crust. You can do it with your hand or using a food processor. Wrap in foil and put in refrigerator for 1/2 hour.

Cut the dough in half and roll to the size of your pan, 18x30 cm (7x12 inches). Place one half in the bottom of buttered and floured pan.

In a large bowl, beat egg whites until stiff peaks form. Add caster sugar and beat until incorporated.

Mix in egg yolks, one by one. Add vanilla extract, chocolate, and lemon juice and mix briefly.

Fold in ground walnuts and gently mix with a spoon or spatula (do not use mixer for this step).

Transfer filling to the pan, cover with second part of the dough. Bake in preheated 175 C (350 F) oven until top is pale golden, about 30 minutes.

Sprinkle with powdered sugar before serving.